1. Technical Field
The embodiments herein generally relate to mobile television (TV) technologies, and, more particularly, to pilot-aided orthogonal-frequency-division-multiplexing (OFDM) systems used in Digital Video Broadcasting-Handheld (DVB-H) systems.
2. Description of the Related Art
Frequency division multiplexed communications systems transmit upwards of thousands of carrier signals simultaneously to communicate information. In the case of OFDM systems the transmitted carrier signals are orthogonal to each other to avoid or minimize mutual interference. In an OFDM system, each transmitted carrier signal may be used to transmit a different unit of data (e.g., symbol) in parallel. In pilot-aided OFDM systems, carriers of the OFDM symbol are modulated, at the transmitter, with data symbols in addition to pilot symbols known to both the transmitter and the receiver. DVB-H is a pilot-aided OFDM system. A subset of the carriers in the OFDM symbol carry pilot symbols known to the receiver in order to be used for channel estimation and correction. The carriers dedicated to the pilot symbols are known as the pilots.
There are two types of pilots, namely, continuous pilots (CP) and scattered pilots (SP). At the receiver, these pilots are used to estimate the channel at the corresponding carrier positions. The pilot positions are not necessarily fixed from one OFDM symbol to another. If the pilot position is fixed across the OFDM symbols, it is called a continuous pilot; if it is variable, it is called a scattered pilot. The continuous pilots occupy fixed locations in the OFDM symbol while the scattered pilots occupy one of four groups of locations depending on the OFDM symbol number. In order to recover the entire channel (all the carriers) from the estimated carriers at the pilot positions, the estimated carriers are interpolated to obtain the entire channel.
In order to estimate the channel (i.e., estimate the channel length, which is the time between arrival of the first and last channel paths at the receiver), the received signal (after taking the Fast Fourier Transform (FFT)) is divided by the pilot symbols at the pilot locations. This gives an initial estimate of the channel at these locations which is then interpolated to obtain the channel at all the active carrier locations. Then, channel correction is performed and a soft estimate of the data can be obtained.
Generally, during channel estimation, perfect knowledge of the scattered pilot locations (i.e., perfect knowledge of the OFDM symbol number) is crucial to the success of the estimation. In practical receivers where time slicing is applied, an offset in symbol number tracking is possible. This results in a wrong estimation of the scattered pilot locations. In order to overcome this problem, a scattered pilot location detection technique is needed which does not depend on the receiver tracking of the OFDM symbol number.